This invention relates to an improvement in the process for preparing urea, in which ammonia and carbon dioxide are reacted in a reaction zone and in which, subsequently, in a stripping zone which is in a heat-exchange relationship with the reaction zone through a wall, at a pressure which is substantially equal to that in the reaction zone, ammonium carbamate present in the synthesis solution thus formed is decomposed, the decomposition products are expelled with the aid of gaseous carbon dioxide, ammonia, inert gas or a mixture of at least two of said substances, and urea is produced.
More specifically, the present process represents an improvement on the basic process described in U.S. Pat. No. 3,356,723 to Kaasenbrood, one of the co-inventors herein, and, more recently to U.S. Pat. No. 3,406,201 to Baumann et al. The disclosures of these patents are hereby incorporated by reference to the extent necessary to understand and further explain the process described herein.
A process of this general type has already been desribed in U.S. Pat. No. 3,406,201 which indicates a "heat integrated reactor-decomposer" system using a stripping treatment using the heat liberated during the formation of ammonium carbamate, to which end the reaction zone is located at the outside of the heat-exchange tubes in which the stripping takes place. Applicants have observed that the manner in which the carbamate is formed in such a reaction zone to a practical degree is not fully explained, for the ammonia introduced into the bottom part of the reaction zone and is only contacted with the gaseous carbon dioxide required for the formation of carbamate and urea in the gas chamber above the tubes. This is because the carbon dioxide is first used as stripping gas in the tubes and, subsequently, after being mixed with expelled carbon dioxide, ammonia and water, flows out of the tubes at the top. Another disadvantage to this known process is that at the temperatures and pressures given, the carbamate present in the synthesis solution can only be decomposed to a sufficient extent in the stripping zone by adding heat to the system from an outside source.